809
FXCA62 TJSJ 212033
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
433 PM AST Sat Sep 21 2024

.SYNOPSIS...

Fair weather conditions are expected to return a few hours after
sunset and continue tonight. A typical pattern of calm nights and
mornings, followed by warm-to-hot afternoons and evenings driven
by diurnal heating and weak steering winds, will prevail in the
coming days. Excessive heat will continue to pose a risk to
vulnerable individuals, particularly those outdoors without proper
cooling or hydration. Additionally, the moderate risk of rip
currents will persist tonight.


&&

.SHORT TERM...Tonight through Monday...

A col area to our north continues to promote light and variable
winds across the islands. The col will shift gradually around the
region through the rest of the short-term period as a broad surface
low pressure lingers to our northeast and a weak surface high
pressure north of Hispaniola extends into the Mona Passage.
Currently, a low-to mid-level high-pressure east of the Lesser
Antilles is providing a weak southerly steering wind flow, but an
ill-defined flow is expected to prevail in general. Therefore, the
diurnal afternoon convection will be driven by a weak sea breeze and
should favor shower and thunderstorm development due to orographic
lift over the central portions of the islands while slowly drifting
over the coastal areas as the sea breeze loses its influence late in
the afternoons into the evening hours. Slow-moving storms will lead
to urban and small stream flooding, with some main rivers possibly
reaching minor flooding at night.

An upper-level trough over the eastern half of the CWA will continue
to move eastward into the Anegada Passage and over the Leeward
Islands during the next 24 hours or so as an upper-level ridge
builds briefly from the west over the region. However, a significant
warming of the 500 mb temperature is not expected, and thunderstorms
are expected to produce frequent lightning again today. On Monday,
an upper-level trough from the western Atlantic is expected to move
over the region, further enhancing shower and thunderstorm
development.

Light winds and relatively drier conditions during much of the
morning and early afternoon will cause excessive heat conditions in
most coastal and urban areas along the islands` lower elevations.

.LONG TERM...Tuesday through Friday...

From previous discussion issued at 512 AM AST Sat Sep 21 2024

The inherited forecast remains on track. According to the latest
model guidance, a deep-layered trough is expected to move just
north of the Caribbean islands until at least late Tuesday into
Wednesday. This weather feature will result in a weak steering
wind flow, with speeds ranging from 1 to 5 knots and gradually
recovering to around 10 knots after its passage. Tuesday is
expected to be a day with variable weather conditions, as the
trough will be closest to the islands allowing the surface
moisture to slightly filter across all levels of the atmosphere.
The 500 mb temperatures are expected to be around -7 to -8 degrees
Fahrenheit on Tuesday, which could lead to an increase in
instability and the potential for thunderstorm development
particularly during the afternoon hours. Despite the passage of
the trough, occasional patches of moisture are anticipated to move
across the forecast area throughout the period, resulting in
showers reaching the coastal areas of Puerto Rico and the Virgin
Islands each day. The strongest shower and thunderstorm activity
is expected each afternoon across the Cordillera Central, with the
activity then drifting toward the coast. Due to weak winds, it is
difficult to determine which area will be more affected, aside
from the interior, which should receive the bulk of the activity
each day. Towards the end of the workweek, a tropical wave will
reach the islands. We anticipate occasional ponding of water on
roads and poorly drained areas from Tuesday into Friday.
Temperature-wise, above-normal temperatures are expected to
persist over the region, with the threat of excessive heat each
day. This heat will mainly impact those without effective cooling,
adequate hydration, or prolonged exposure.


&&

.AVIATION...
(18Z TAFs)

SHRA/TSRA with mtn obscurations and MVFR/IFR conds over interior and
western PR should last till arnd 21/21Z. This may cause tempo MVFR
to brief IFR conds at TJSJ/TJBQ late in the afternoon. Across the
USVI terminals mostly -RA/VCSH should prevail thru mid-afternoon.
Winds less than 10 kt with a few higher gusts in mostly sea breezes
and near TSRA. SHRA/TSRA ovr land expected dissipate around 21/23Z.
VFR conditions should prevail overnight thru early Sunday afternoon.


&&

.MARINE...

A weak surface low pressure to the northeast of the region will
maintain weak and variable winds into the early part of next week.
Showers and isolated thunderstorms will persist across the regional
waters, particularly off the western and southern coastal waters of
Puerto Rico each afternoon.

&&

.BEACH FORECAST...

The moderate rip current risk will persist for the northern and
eastern coast of Puerto Rico and Culebra tonight. Elsewhere is
low. The risk will drop to low tomorrow, but a moderate risk will
return by midweek.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001-
     003-005-007-008-010-011.

     Heat Advisory until 5 PM AST this afternoon for PRZ002-004-012-
     013.

VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002.

     Heat Advisory until 5 PM AST this afternoon for VIZ001.

AM...None.
&&

$$

SHORT TERM...DSR/RAM
LONG TERM....ICP
AVIATION...DSR/RAM
PUBLIC...GRS/ICP